1. Field of the Invention
The present invention relates to a solid preparation produced for improving the dissolution of a poorly soluble drug, particularly to a solid preparation comprising a solid dispersion that can be rapidly disintegrated and that allows a drug to be dissolved.
2. Description of the Related Art
Poorly soluble drugs have high crystallinity and extremely low solubility in water. Thus, bioavailability or internal absorption of preparations produced from these drugs is low so that there is a problem in that the drug action is insufficient. As a technique for addressing this problem, a solid dispersion has been developed in which molecules of a poorly soluble drug are dispersed in a polymer carrier such as a cellulose derivative, in an amorphous state.
Conventional solid dispersions are used as preparations in the form of capsules containing a solid obtained by spray-drying a cosolvent in which a poorly soluble drug and a carrier are dissolved, or in the form of fine granules or granules as they are. The form of tablets, which is a generally used dosage form of solid preparations, is most preferable because tablets are easily prescribed and administered in a fixed dose, and easily handled and taken by patients.
It is known that in the case of tablets produced from a solid dispersion powder, the porosity of the tablets is often lowered not only due to a reduced specific surface area, but also due to plastic deformation of amorphous drug molecules during a compression-molding process and strong bonding between carrier polymers. This low porosity leads to slow permeation of water molecules into the tablets in administration, and to slow disintegration of the tablets, and thus the solid dispersion cannot exert its original effect of improving the dissolution. Furthermore, the viscosity of a water-soluble or enteric polymer serving as a carrier increases during hydration or dissolution, and thus a type of hydrogel layer is formed on the surface of the tablets during dissolution, so that water is further prevented from infiltrating.
As means for addressing these problems, Japanese Patent Application (PCT National Phase) Unexamined Publication No. 2005-517690 has proposed a tablet comprising a solid dispersion powder, a disintegrator and an excipient comprising porosigen, and obtained by spray-drying. Furthermore, Japanese Patent Application Unexamined Publication No. 5-262642/1993 has proposed a powder in which a water-soluble polymer base and if necessary, an excipient and a disintegrator are added to a poorly soluble drug. However, an added amount of a concentration-enhancing polymer or a water-soluble polymer base, serving as carriers, is large, and thus the rate at which a drug is dissolved tends to be lowered. Furthermore, in the case of a solid dispersion powder obtained by spray-drying as in Japanese Patent Application (PCT National Phase) Unexamined Publication No. 2005-517690, it is necessary that after the solid dispersion powder is mixed with the other ingredients, the mixture is compressed and pulverized for formation of a granulated powder for tableting. The particle size of the solid dispersion powder prepared by spray-drying in this manner is small, and thus when it is simply mixed with an excipient, segregation is caused so that ingredients are not distributed uniformly in the powder for tableting. Moreover, this process makes the operation complicated, and the solid dispersion may be recrystallized in compression. Furthermore, the disintegrator is added after the solid dispersion has been prepared, and thus when the solid dispersion is aggregated and bonded in the tablet due to high bonding strength of the carrier, aggregation may be formed and dispersed in water during disintegration, lowering the dissolution of the drug.
Japanese Patent Application Unexamined Publication No. 2004-67606 has proposed a tablet comprising fine granules obtained by spraying a solution containing itraconazole, which is a poorly soluble drug, a water-soluble polymer and an enteric polymer, on a mixed powder of an excipient and a disintegrator, granulating and drying the resultant. However, the amount of the disintegrator added is small and it takes as long as 360 minutes for the drug to be dissolved from the tablet. Thus, the disintegratability of the tablet is not improved.
Hirasawa et al. (Journal of the Pharmaceutical Society of Japan, 124(1), 19-23 (2004)) has proposed a tablet produced by loading an ethanol dispersion as a binding liquid containing nilvadipine which is a poorly soluble drug, crospovidone and methylcellulose, into a mixed powder of materials such as lactose, methylcellulose and low-substituted hydroxypropylcellulose, and agitating and granulating the mixture. The nilvadipine is soluble in ethanol, but crospovidone and methylcellulose are not soluble in ethanol. Thus, it seems that the ethanol functions only as an agent for dispersing and diluting amorphous nilvadipine because a co-dissolved state is not obtained. In order to disperse amorphous drug molecules in a polymer serving as a carrier, it is necessary to obtain a co-dissolved state in a cosolvent in which both the drug molecules and the polymer are dissolved. Thus, it seems that the solid dispersion of amorphous nilvadipine described in Journal of the Pharmaceutical Society of Japan 124(1), 19-23 (2004) does not have sufficient solubility. Furthermore, since an amount of water-soluble polymer added is large, it may be difficult to obtain a preparation that can be rapidly dissolved.